1. Field of the Invention
The present invention relates to an image forming apparatus and a method of forming an image.
2. Description of Related Art
In an electrophotographic image forming apparatus, a developing device is employed in which the surface of a photoconductor drum is charged, and the charged region is exposed to light in accordance with an image data to form an electrostatic latent image, and the electrostatic latent image thus formed is developed to visualize (develop) the image.
In such a developing device, a two-component a developer containing carrier and toner is used, and the toner is frictionally charged and then absorbed by the electrostatic force of the electrostatic latent image formed on the surface of the photoconductor drum, whereby the electrostatic latent image is developed to form a toner image.
A developing roller of the developing device includes therein a plurality of magnetic poles, and a cylindrical developing sleeve which is rotatably supported on the outside of the developing roller. The developing roller conveys to a development region the carrier on which toner is attached, while holding the carrier on the developing sleeve, so as to perform development.
Incidentally, there has been a problem that, in the case where solid image parts 10 and non-image parts 20 are formed side by side on the leading-edge side of the image in the rotational axis direction of a photoconductor drum and thereafter halftone image 30 having a large area is formed as illustrated in FIG. 1, the image density of halftone image 30 becomes nonuniform under the influence of the image (solid image parts 10) of the previous rotation on the developing roller. This problem is known as uneven image density called “development memory.”
Now, the mechanism of causing the development memory is described in detail. When an image partially having a high density part (solid image part 10) is printed, the height of the toner layer attached on the surface of the developing sleeve in the region corresponding to solid image part 10 (hereinafter referred to as “region A”) is lower than that in the region corresponding to non-image part 20 (hereinafter referred to as “region B”). To be more specific, on the surface of the developing sleeve, the toner is attached in region B whereas the toner is not attached in region A. As a result, on the surface of the developing sleeve, the potential of the toner layer in region A is lower than in region B. In such case, when halftone image 30 having an uniform image density is printed after a potential difference is caused on the surface of the developing sleeve, the development property of toner on the photoconductor drum differs between region A and region B even when the same developing bias is applied thereto. That is, as illustrated in FIG. 1, under the influence of the image (solid image parts 10) of the previous rotation on the developing roller, portions 40 corresponding to solid image parts 10 undesirably have an image density lower than that of portions 50 corresponding to non-image parts 20 in halftone image 30 printed on recording sheet 60. In order to cancel this image density difference, the potential of the toner layer has to be uniformized by maintaining the state where the toner is uniformly attached on the surface of the developing sleeve or the state where no toner is attached on the surface of the developing sleeve, at the time of developing the electrostatic latent image on the surface of the photoconductor drum.
Japanese Patent Application Laid-Open No. 8-54787 discloses a technique of development in which a sleeve is rotated at least one revolution to remove the sleeve contamination before developing an electrostatic latent image only in the case where electric charge of a high charge amount is generated in developer due to a low humidity and thus the risk of sleeve contamination is raised. According to the technique disclosed in Japanese Patent Application Laid-Open No. 8-54787, when the sleeve rotates, rubbing between the developer attached to the sleeve and the developer in a development section removes the sleeve contamination.
Japanese Patent Application Laid-Open No. 2006-145894 discloses a technique of facilitating uniformization of the amount of the developer and the charge amount of the developer of a developer layer formed on a developer bearing member by limiting the ghost due to the development history on the developer bearing member. The developing device disclosed in Japanese Patent Application Laid-Open No. 2006-145894 includes: a rotatable developer bearing member which faces an image bearing member on which an electrostatic latent image is borne, bears on its surface the developer, and has an arithmetic average roughness Ra of 0.7 [μm] or less; a developer supply member which is disposed separately from the developer bearing member and to which a supply bias for facilitating supply of developer to the developer bearing member is applied; and in addition, a developer collecting member which faces the developer bearing member and applies a voltage to collect the developer remaining on the developer bearing member.
Japanese Patent Application Laid-Open No. 2008-224912 discloses a technique of preventing development memory regardless of the property of the developer to be used in the case of hybrid development. In the technique disclosed in Japanese Patent Application Laid-Open No. 2008-224912, a supply-magnet roller for forming a toner layer on a developing roller is oscillated to increase the scraping force for scraping the toner attached on the surface of the developing roller by a magnetic brush, thereby increasing the efficiency of collecting the toner remaining on the surface of the developing roller.
However, the problem with the technique disclosed in Japanese Patent Application Laid-Open No. 8-54787 is that, rubbing between the developer attached to the sleeve and the developer in the development section results in degradation of the developer, shortening the life of the developer.
In addition, the problem with the technique disclosed in Japanese Patent Application Laid-Open No. 2006-145894 is that, particularly in the case where a toner having a charge amount equal to or greater than 50 [μC/g ]is used, a part where the toner remaining on the surface of the developer bearing member can be completely removed and a part where the toner remaining on the surface of the developer bearing member cannot be completely removed may both be formed. Further, in the case where the toner remaining on the surface of the developer bearing member is not electrostatical and has a strong attaching force, removal of the remaining toner with use of electric field is difficult, and the part where toner can be completely removed and the part where toner cannot be completely removed may both be formed easily. In such case, undesirably, the development history on the developer bearing member cannot be sufficiently limited.
In addition, the problem with the technique disclosed in Japanese Patent Application Laid-Open No. 2008-224912 is that, in the case where the charge amount of the toner attached on the surface of developing roller is greater than 50 [μC/g], the attaching force of the toner attached on the surface of the developing roller is strong, and therefore the toner cannot be sufficiently removed from the surface of the developing roller by only the scraping with the magnetic brush.
Since the techniques according to Japanese Patent Application Laid-Open Nos. 8-54787, 2006-145894 and 2008-224912 have the above described problems, the techniques cannot be used as they are to solve the problem that the development memory occurs due to non-uniform potential in the toner layer on the surface of the developing sleeve.